This invention relates to systems for locating transponders or reflectors (targets) in space by receiving and processing energy reflected or transponded from the targets, and more particularly to a system for extracting timing information required to locate targets directly from energy received from two or more targets when the transmitter of the energy is not detectable or is not processed at the receiver.
For many years, engineers have employed "bistatic" processing to locate energy emanating from a point in space in response to illumination by energy from a transmitter offset from the receiver. In one example, a ground-based scanning radar illuminates an airborne vehicle and energy emanates from the vehicle in the form of either reflections or separate emissions from an onboard transponder triggered by receipt of the radar energy. The energy emanating from the airborne vehicle is received and processed at a receiver removed an arbitrary distance from the ground-based radar. The only requirement on the receiver site (which can be an airborne site) is that both the transmitter site and the airborne vehicle must be close enough to the receiver site for energy to be detected and processed at that site.
Conventional bistatic processing techniques require the measurement of the difference in time of arrival of energy from the transmitter relative to energy emanating from the target. Knowledge of the differential time of arrival (DTOA) and the location of both the receiver and the transmitter is used to place the target on an ellipse of revolution (ellipsoid). Any of a number of techniques can be used to determine the direction of arrival (DOA) of the target energy relative to either the receiver or the transmitter (or both). The intersection of the ellipsoid and the DOA form an unambiguous estimate of the location of the target.
The bistatic process described above suffers from one significant drawback: the receiver must process energy received directly from the transmitter. If energy is transponded from a transponder in space, this requirement generally leads to the need for two receivers, because the transponder is generally at a different frequency than the transmitter. An even more restrictive implication is that the signal from the transmitter must be detectable and processable at the receiver. If the transmitter is shadowed by a mountain, for example, target energy may be readily processed at the receive but bistatic processing is not possible because direct transmitter energy is not available at the receiver.